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What is the procedure for an engine fire in the 737-800?
If there is a fire in one of the engines of the 737-800, what is the procedure to be followed in the following situations?

Climb
Cruising altitude
Approach

What should the pilot do in the cabin in these three situations? In the climb stage, should the plane return to the airport of origin or continue the flight?

In general, you always follow the steps in the Quick Reference Handbook (QRH) after any failure or other problem. An engine fire is serious enough that it requires Memory Items, which are performed immediately before getting the QRH. In this case, the memory items are the first 5 steps of the QRH checklist:

ENGINE FIRE or Engine Severe Damage or Separation

Autothrottle (if engaged) . . . . . . . . . . .Disengage
Thrust lever (affected engine) . . . . . . . . Confirm . . . . . Close
Engine start lever (affected engine) . . . . . . . . Confirm . . . CUTOFF
Engine fire switch (affected engine) . . . . . . . . Confirm . . . . . . Pull
To manually unlock the engine fire switch, press
the override and pull.
If the engine fire switch or ENG OVERHEAT light
stays illuminated:
Engine fire switch . . . . . Rotate to the stop and
hold for 1 second
If after 30 seconds the engine fire switch or ENG
OVERHEAT light stays illuminated:
Engine fire switch. . . . . . . .Rotate to the
other stop and
hold for 1 second
If high airframe vibration occurs and continues
after the engine is shut down:
Without delay, reduce airspeed and descend to
a safe altitude which results in an acceptable
vibration level.
If high vibration returns and further airspeed
reduction and descent are not practicable,
increasing airspeed may reduce vibration.
ISOLATION VALVE switch . . . . . . . . . . . . CLOSE
PACK switch (affected side) . . . . . . . . . . . . . OFF
This causes the operating pack to regulate to
high flow in flight with the flaps up.
APU BLEED air switch . . . . . . . . . . . . . . . . . OFF
Choose one:

APU is available for start:
APU . . . . . . . . . . . . . . . . . . . . . . START
When APU is running:
APU GEN switch (affected side) . . . . . . . . . . . . . ON
APU is not available:
Go to step 11

Balance fuel as needed.
Transponder mode selector . . . . . . . . . . TA ONLY
This prevents climb commands which can
exceed single engine performance capability.
If wing anti–ice is needed:
ISOLATION VALVE switch (after fire has been extinguished). . . . . . AUTO
Plan to land at the nearest suitable airport.

(Boeing 737NG QRH 8.2)
You can see that the items 2 to 4 have a Confirm written before the actual action. This means that the Pilot Flying (PF) will put their hand on the respective lever, then the Pilot Monitoring (PM) confirms that this is indeed the correct one for the engine that has a fire and says "Confirm" and only then the PF will actually perform the action. This is to make sure that you don't accidentally shut down the operating engine. After step 5, the engine with the fire indication is shut off and the fire should be extinguished.
After performing the memory items, the PM would get the QRH out and the pilots will work through the checklist (including verifying that steps 1-5 have been performed correctly).
The steps do not depend on the situation (climb, cruise or approach). You always follow the above procedure. The general order of importance is Aviate, Navigate, Communicate: first you keep the aircraft flying, then you figure out where you are going and where you want to go, then you communicate with ATC.

What should the pilot do in the cabin in these three situations?

I'm not sure what you mean by this. The pilots are in the cockpit and after figuring out what to do next, they will communicate with the cabin crew and the captain will probably make a PA to the passengers.
If you refer to pilots flying as passengers in the cabin: They would probably do nothing. A single engine fire is trained regularly by the flight crew and the two pilots in the cockpit should have no trouble handling the situation without help of a third pilot.

And at the climb stage, should the plane return to the airport of origin or continue the flight?

You can see in the checklist that step 14 says: "land at the nearest suitable airport". This is not as strict as "land as soon as possible", where you should get the aircraft on the ground without any delay (smaller airport, military airport, whatever). The nearest suitable airport should be a civilian airport within normal descent range where a landing can be performed without further risk. In the initial climb stage, this would typically be the departure airport.
Continuing the flight to the destination should only be done when the aircraft is already close to the destination anyway (e.g. during descent). An engine fire during cruise may require diverting to a different suitable airport.

“at nearest suitable airport” is less strict than “land as soon as possible” mainly in that you are not supposed to do emergency descent. So if there are multiple airports with sufficiently long runway, adequate fire-fighting capability and comfortably above weather minima within the normal descent range, you can choose freely among them, but you are not allowed to look any further away.

@JanHudec You're right, I updated the answer. I had that mixed up with land as a soon as practical.


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How to increase payload capacity of plane?
Keeping thrust, wing span, weight (of plane) constant, can we increase the payload capacity (nearly 50% more)? Adding payload to plane, it decreases thrust/weight ratio, how does it affect the control over plane?

You can generally drain some fuel out of the tanks and still have useable range but recoup the fuel weight as payload capacity.

Are you asking about how to improve the general design, or how to improve a given airplane standing in front of you?

How to improve the plane standing front of you , keeping thrust , wingspan and weight of plane same. I can change wing area , airfoil shape , fuselage or something else. This is a conceptual question. Not the practical one.

If you need to keep the airplane weight the same, I do not see how a 50% payload increase is realistic unless the initial payload was minuscule in both mass and volume. If weight is constant, no change in control is to be expected.

You can increase the wing area and airfoil shape by dropping the flaps.

I'm not sure what you mean by 'payload capacity'.  Airplanes have what's termed a 'useful load' which is the maximum combined weight of the passengers, baggage and cargo, and useable fuel.
If you wanted to increase useful load, you would have to make the airframe and all systems aboard the airplane which constitute useful load lighter.  This does have the adverse effect of making the structure weaker, lighter engines generally are less powerful so you have less thrust, and removal of mission systems reduces aircraft capability.  Manufacturers strive to address these things and improve them over time with more rigid aluminum alloys or composites for increased strength to weight ratio, higher thrust to weight ratio engines with better fuel efficiency and lighter mission systems.
How an increase in weight affects aircraft performance not only depends on how heavy the airplane is but where the center of ends up being located.  On a conventional airplane a forward CG produces better stability but requires more lift from the wings, resulting in decreased performance.  An airplane with a more aft CG offers better performance, but is more unstable, particularly during high alpha operations such as slow flight.   The maximum takeoff weight of the airplane was established during preliminary design studies and verified during flight testing so that it cannot exceed the load limits of the airframe and offer the required performance over a wide range of potential operating conditions.
Aircraft are, on occasion, approved via a Supplemental Type Certificate (STC), to increase their useful load, if the OEM or third party vendor can demonstrate that the mod can still operate safely and fulfill the regulatory requirements.
At any rate, the heavier the airplane is, the more lift it needs to fly, which means more drag, which means more thrust, which means greater fuel burn.  This is why the airlines are so miserly in issuing out the bare minimum gas to meet federal Part 121 and company SOP requirements for their flights.  The fuel saved doing this saves the company millions of dollars annually.

I'm assuming this is for something like the BMFA Payload Challenge where you design a model plane to carry a payload.
You need to increase the wing area, as lift is a function of wing area and speed (and speed requires power, so you can't increase it).
As your span is limited, you can add chord, or add another wing (i.e. turn it into a biplane).
More lift will create more induced drag, and as your power is constant, the result will be a lower flying speed, so you might need more than a 50% increase in area to generate 50% more lift.

how does it affect the control over plane ?

When you're flying slower, the wind has more effect. Also your control surfaces will generate less force, so your controls will feel less responsive.

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How long does it take for an airline to establish a new international destination/route?
This post discusses the process of opening a new route. I wonder how long does it take for an airline to get permission to fly a new route. I am interested on this because, say an airline goes bust during the current covid-19 crisis. If another airline wants to fly a route that was previously flown by the busted company, it will take some time for that to happen. Naturally, conditions might be particularly different in the current situation but it would be interesting to know the length of the process in normal times.
This article says it might take "years to develop". Anyone with a more specific estimate?

There is no better estimate. Every possible route comes with numerous hurdles to overcome. Some routes will be resolved quickly, others take longer.

related: How much time does it takes to buy a landing slot?

In Australia:
Our major airlines quality for what is called an "area AOC". That means they already have permission to operate ad hoc services to any port in Australia and in fact most other countries. All they need to do is an internal risk assessment specific to the new port and address any findings from that (e.g simulator sessions, maybe an on-site inspection), and share it with the regulator. This process can take as little as a few days if it's a simple well-established airport.
If they want to start new regular or RPT services, it's a little more complex in that they need to have a line maintenance facility at the new port, or contract another company to do it for them. This facility is inspected and approved by the regulator which can take a few months to arrange.
You also need government approvals to operate the route, both for the arrival country and overflight arrangements (though typically those are already in place from existing routes). Some countries can be a beaurocratic nightmare and take months to give this, others much quicker. Airports are similar, negotiating parking fees and reserving slots if required.
So to summarize, approval for a one off charter to a familiar port can usually be arranged within a week or two, a regular service needs a couple of months. Of course it can vary wildly depending in the scope of the operation, and I'm ignoring the sometimes years of economic research and monitoring that goes into deciding whether a route is even financially viable.

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On an energy-maneuverability (E-M) diagram how does the shape of a specific excess power (Ps) curve change as thrust is increased or decreased?
Most E-M diagrams for high-performance aircraft show a "dog house" plot with turn rate on the Y axis and velocity on the x axis.  They may also have additional lines for constant turn radius and load factor.  There are also Ps curves that show specific excess power at an given point in the performance envelope.  The Ps curves tend to be shaped like a "rainbow" when superimposed on this chart.  Most curves that I've found only demonstrate performance at maximum thrust (normally maximum afterburner).  However, I'm curious what the Ps curves would look like at 100%, 75%, 50% power, etc.  I theorize that the Ps curves would tend to become flatter and would shift to the left of the chart.  Ideally, I'd like to see a comparison of two charts at different (known) throttle settings (with all other parameters being equal).

Why would curves for less than mil power be interesting in any way?

The greater power need to hold the velocity in the turn, the less excess power you have.  A tighter turn will be draggier.  The curves at lower power levels will look the same as max power for progressively smaller engines, all the way down to hamster wheel.

The shape does not change much, but they shift up and down. Below you see a diagram I once made for a HALE unmanned airplane with a turbofan engine. Instead of varying thrust I calculated the instationary turn rate at different sink speeds. Sinking frees up energy similar to a thrust increase, so these curves should look the same for different thrust levels. After all, the unit for SEP is a speed, too.

Turn rate diagram for different sink speeds and flap settings. Blue lines are for cruise and red curves for the landing setting of a simple camber flap. The aerodynamic model did not include wave drag, so the results above Mach 0.7 are not correct.
As you rightly assumed, the curves shift to lower speeds for lower thrust, but only the red ones become flatter for lower sink speeds / thrust levels.


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Why does the 737-300 outflow valve indicator give different readings?
The 737-300 outflow valve opens and closes in manual mode under AC or DC side. While in DC, the indicator is accurate, but when it's in AC mode it reads open when closed.
Does the outflow valve have separate potentiometers for indicating both AC and DC?

Welcome to [aviation.se]. Could you clarify - is this a theoretical problem, is it a problem you've actually seen in an aircraft you've flown, are you an A&P trying to resolve this issue? You can [edit] your question to include this info. After you've done that, take the [tour] to familiarize yourself with how this forum works, since it's a bit different than a run-of-the-mill discussion board.


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What would be the minimum number of engines required to climb in a 747?
There was a really good Air Crash Investigation by Nat Geo which showed the British Airways Speedbird 9 issue where volcanic ash swept over all four engines and through the air vents inside the cabin. As a result, all engines stopped working.
After around fifty attempts to restart the engines, engine four roared back to life. Although this did bring good news to all onboard, one engine would still not have given them enough power to clear the mountains that were in their path around Jakarta. Due to this, the Captain turned back to sea.

Nat Geo Flight 9. All engines stopped.
I don't think National Geographic would be super accurate but this looks like one engine could have cleared this mountain.
Anyway, this got me thinking.
A 747 can fly on one engine but can't climb? This does make sense, one engine can make a 747 fly for longer but it cannot climb with just one. What then would be the minimum number of engines required for the jet to clear the mountains? Two?

@Michael Hall said it best, in the first sentence of item 1 of his answer:

There are simply too many variables for a definitive answer.

Here are two extremes to illustrate the range of answers insofar as altitude is concerned:
Let's say you have a 747 at it's optimum altitude in cruise with all four engines operating, say 37,000 just for the heck of it. Now let's say you lose one engine. Can you now climb on three engines? No, in fact you won't even be able to keep the altitude you have. You will start drifting down.
Now let's say that same 747 with the same load (actually heavier because it would still have the fuel on board that it used to get to its 37,000 foot cruise) lost an engine while on the takeoff roll but after V1. Will it climb? Yes, even with the drag from the gear and takeoff flaps you will be able to lift off the runway and climb.
But back to the one-engine scenario. My gut level feeling is that any 747, if carrying no passengers or cargo and only, say, 30,000 lbs of fuel and down low (5,000 ft?) and in a clean configuration could eke out a little climb. Can't prove it though.
But start adding weight or drag and the answer would change. The first model to not be able to do it with weight and drag being added would be the 747-100. The first -100s I flew had a max takeoff weight of 735,000 lbs as I remember. The next model to fail would be the -200. 825,000 lbs was the highest max takeoff for the -200s I flew. The highest max takeoff that I have for a -400 on the DOS weight & balance software I wrote was 910,000 lbs.
The biggest difference performance-wise is due to engine size. The more power the one engine has, the more it can keep aloft.
And there are other variables. What was the nature of the failures of the three engines? Are they windmilling or seized? Even whether the remaining engine is an inboard engine (2 or 3) or an outboard engine (1 or 4) is a factor. When it comes to performance, even how many insects are splattered against the leading engine of the wing can make a difference.

Can it trade some speed for altitude maybe if low enough and fast enough?

@h22 Yes, but if operating with asymmetrical thrust, you'd have to be very careful doing that lest you set yourself up for a stall of one wing.

I would also wonder about how well the working engine is running... having just been filled with ash it may be experiencing sub-optimal performance.

We use the polar of the 747 as given in this answer:

Let's suppose that it took off at its MTOW of 377'000kg and that the accident happened somewhere midway i.e. with half of its 198'000 liters of fuel burned: this corresponds to some 0.5x198'000x0.82=81'000kg and therefore to a total weight at the time of the incident of 377'000-81'000=296'000kg=2'903kN.
According to the captain's website, engines number 4 and 3 restarted somewhere at around 12'000ft ($\rho$=0.75kg/m³) and at a speed of some 300kts=154m/s. With this data we get a lift coefficient of:
$C_l=\frac{W}{½\rho V² S}=\frac{2'903'000}{½0.75\times 154² \times 511}=0.64$
According to the polar, this corresponds to a drag coefficient of about 0.038 which gives a drag i.e. a needed thrust of:
$D=½\rho V² S C_d=T=½0.75 \times 154² \times 511 \times 0.038=173kN$.
Each B747-200 turbofan delivered 243kN of thrust at sea level which became more or less one half at 12'000ft i.e. 120kN. So one engine was definitely not even enough to win drag and keep the speed while two were enough not only to win drag but also to gain some altitude. This result perfectly matches the incident's description: "engine number four finally started, and at 13:56 UTC (20:56 Jakarta time), Moody used its power to reduce the rate of descent. Shortly thereafter, engine three restarted, allowing him to climb slowly".

These calculations are very close to using the 747 best glide ratio of 17:1 at around 300 knots: 2903 kN/17 = 171 kN.  Must have taken a cool head to glide it while trying to restart those engines.

@RobertDiGiovanni: yep I think so. But apparently the alternative would have been to ditch in the ocean

There are simply too many variables for a definitive answer.  It depends greatly upon weight, how high the mountains are, starting altitude, and distance available to climb.
1800 feet seems much lower than necessary.  Under normal conditions in an unpressurized aircraft with passengers you would want to get below 10,000 feet.  However, in an emergency where terrain is a factor people will likely remain conscious at 10,000 to 18,000 feet.  Above that useful consciousness decreases rapidly.

P.S.  For my understanding, is your comment "this looks like one engine could have cleared this mountain" based on your analysis of the picture you posted?!


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What are the most maintenance-intensive parts of a helicopter?
Which components of a helicopter are the "driving" ones that require most regular attention and down-time, and how often do they last between inspection or overhaul? I don't have a specific model in mind, but something along the lines of a 4-passenger light transport heli
Along with this, are there developments that might increase these intervals, and how much is defined by the manufacturer vs regulatory bodies (FAA/CAA/EASA etc.)?

This question is a bit to broad as it stands. The answer likely varies by make/model so if you narrow it down a little bit you will get far better answers. There are also 3 questions here and generally this site sticks to one question per post. All of them are valid (except maybe the last) and could be split up.

I would say pilot. Legally down after 8 hours of operation per calendar day. Unpredictable downtime after a good day after the ball game etc.

@vasin1987 Perhaps, but their swap time is relatively minimal, and they can be overhauled outside of your mechanics' working hours

the question is a bit broad but in general, 1 any unsealed Bering's maby be as short as 5 hours, oil filters/air filters 10-30 hours, and so on. In general the intervals are set by the manufacture after testing approved by the regulator, once the manufacture sets the interval, it becomes mandatory [very generally]. you could try the R44 manual: https://robinsonheli.com/r44-maint-manual/

The pilot? Need maintenance every a few flight hours?

The question is a bit broad but in general but I will do my best.
First we must look at how maintenance intervals are determined, normally during the initial certification the manufacture testes there aircraft and determines its limitations both in flight, and for maintenance. the regulator generally looks over testing, and documentation and decide if they will approve the 3 parts: the Aircraft, the Flight Manual, and the Maintenance manual. generally the regulator will not do there own tests, they just decide if the testing carried out is sufficient. (this is my best understanding of this, its complicated and a little different depending where/when the aircraft was developed)
Operators who have larger fleets of 1 type of aircraft, or large maintenance/overhaul facility's  will sometimes develop there own maintenance manuals, during these processes they will sometimes demonstrate to the regulator that with newer testing methods etc. that certain components are actually lasting longer than originally proven. Upon approval of there regulator these new documents become law. Occasionally these changes are driven by the manufacture, often when competing for larger contracts ie. Military that wants different maintenance.
As far as "driving" components most people would think the large items requiring very long overhauls, Ie. rotor blades, transmissions and engines.
To answer your questions on shortest maintenance intervals, older helicopters often have small field serviced items such as vibration damping oil struts, or unsealed bering's that may need to be serviced every day or more. Astars air filters need to be cleaned and oiled something like every 15 hours, and piston engines often need oil filters every 25 hours or less. most of these jobs only take an hour or two.
As far as inspections, all helicopters have a daily inspection that is carried out every day checking for issues.


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Why do helicopter rotors have constant section and angle of attack?
As I understand it helicopter rotor blades keep a constant aerofoil section along their length, and are not twisted. Other rotating aerodynamic devices such as propellors, turbofan fans and wind turbines thicken and get a steeper angle towards the hub due to the lower incident airspeed (in the direction of rotation).
I'd guess it'd have something to do with variation of airspeed on the advancing and retreating sides maybe?
Edit: some are pointing out that this may in fact not be true, and that rotors can in fact have twist, just that it's less visually apparent. The chord at least would still appear to be constant
Edit 2: Since posting this I've taken a module in helicopter aerodynamics at uni, and the question is indeed wrong, blade twist is most definitely a feature of helicopter design.

Very interesting question. Can you please point out to any references for no-twist and single airfoil rotors? It’s a surprise for me to hear that a helicopter rotor has no twist.

I will suggest that you heard incorrectly.  When you assume something, and then ask "Why is it this way?"  you end up asking an an invalid question. It's a bit liked asking "have you stopped beating your wife?" -- it assumes an answer in the asking.   A better question is "do helicopter rotors have a constant section and angle of attack or are they twisted?'  That way you get an answer to "what you heard' rather than assuming that it is so.  I'll also point out that each helicopter's rotor blade is optimized for that helicopter ...

Random model (Sikorsky S-76): "The main rotor blades use an SC1095 airfoil and have a non-linear twist to load the blades evenly in a hover. Tip caps are swept to reduce noise and vibration", source.. As the question is based on a wrong assumption, it will be difficult to answer properly.

For blades with highly visible twist and nonconstant chord, see NASA's Ingenuity.

Helicopter blades do have twist. The relative air speed increases from blade root to blade tip, and therefore on an untwisted blade lift would increase quadratically from root to tip. Or going the other way, lift would reduce quadratically from tip to root. Blade twist fixes this: it recuperates the lift generated on the inboard section of the rotor.
From Ray Prouty, Helicopter Aerodynamics Vol. 1., page 248:

By twisting the blades to reduce the lift at the tip, the flow pattern can be made more uniform and the hover figure of merit can be improved by up to about 5%. Since this may represent a 20% increase in payload and since twisted blades are about as easy to manufacture as untwisted blades, there is a good reason to use twist - and all modern helicopters do.

Due to the aerodynamic lift equation, ideal blade twist would be non-linear: again, would vary quadratically with blade span. But this ideal twist is only ideal in the hover situation, and definitely not ideal in the factory when the blade needs to be produced.
Pouty's article is very much worth reading, and further mentions:

High twist produces vibrations by the centre of pressure traveling a lot during rotation.
Negative blade twist (tip angle lower) is good for retreating blade stall but bad for autorotation, which would be helped by positive blade twist.
Optimum twist differs for hovering In Ground Effect and Out of Ground Effect.

As other answers have said, manned and large drone helicopters have twisted rotor blades.
RC helicopters almost all have zero-camber (symmetrical), zero-twist rotor blades.  These are almost comically inefficient (the center half of the disk produces almost no lift), leading to typical flight times of five minutes or so.  They have the benefit of working just as well upside-down as rightside-up.  "3D" maneuverability is critically important for this consumer base.  If you watch videos of these things doing stunts, you'll often see the helicopter bouncing back and forth between positive and negative lift in a symmetrical fashion.  This isn't possible with a twisted, cambered airfoil.

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Why do 20-series Learjets have such an extreme susceptibility to mach tuck?
Mach tuck is a change (usually nosedown) in an aircraft’s pitching tendency during transonic flight relative to its pitching tendency in below-transonic flight, caused by the appearance of areas of supersonic airflow (with attendant formation of shockwaves, sometimes accompanied by regions of flow separation) around the aircraft when it exceeds its lower critical mach number.  It first showed up immediately prior to and during World War II, as a completely unexpected, and quite deadly, type of loss of control at high speeds (especially on aircraft using the very thick airfoils then in fashion); however, by the mid-1950s, advances in wing and airframe design had reduced mach tuck to little more than a nuisance, allowing even first-generation jetliners to safely fly at transonic speeds without risking a loss of control.1
In contrast, 20-series Learjets, although dating from the 1960s (by which point we already knew very well how to make aircraft highly mach-indifferent), tuck like an aircraft from the 1940s, pitching down suddenly and violently at approximately mach 0.81.  This extreme susceptibility to mach-induced loss of control at high speeds caused a rash of crashes in the 1970s and 1980s, and is one of the many factors that make first-generation Learjets some of the most unforgiving aircraft ever sold to the public (at least, if the NTSB’s AAR archive is anything to go by).2
It is quite hard to see what about these aircraft would cause them to have such evil overmach characteristics; admittedly, they do have straight, rather than swept, wings, but this should impact more where the lower boundary of the transonic flight regime is - not so much how the aircraft behaves once it’s already in transonic flight - and, besides, the thinness of the wings should compensate for their lack of sweep.3
What is the cause of the extreme susceptibility of first-generation Learjets to mach tuck?

1: First-generation jetliners are, nevertheless, generally kept below their lower critical mach numbers in normal line operations, due to the greatly-increased drag associated with transonic flight; however, further design advances, first seen on the Convair 990, greatly mitigated even the transonic drag penalty, with the result that second-generation and later jetliners routinely cruise at transonic speeds.
2: Other factors included a balky autopilot, spoilers that make the aircraft go faster when extended (by causing a large pitch-down tendency of their own), and the fact that their original certificated service ceilings were well into the coffin corner.
3: The reason swept wings help with transonic flight is that they make the wing appear, to the air flowing over it, to be thinner than it actually is, thereby helping to delay and mitigate shockwave formation and flow separation.  These same benefits can also be gained by actually making the wing thinner.

The early Learjet wing was taken from a Swiss fighter-bomber design from the 1950s which was designed when our understanding of transsonic aerodynamics was still developing. And Lear was pushing this design to its limits.

The 20 series early wings use a 64A109 airfoil. The P16 uses a 64A110.

The early series 20 Lears have always had boundary layer separation issues, so much so that any time the wings are removed and reinstalled the aircraft must be test flown. Mach tuck is an issue with all aircraft capable of trans-sonic flight, but on the early Lears this is apparently exacerbated by the susceptibility to boundary layer separation from the laminar flow airfoil.
To address this and other shortcomings in the 20-series wings, a number of modifications have been developed. Vortex generators were installed in front of the ailerons, and then a boundary layer energizer "fence" was developed. Dee Howard and Gates Learjet jointly developed a "cuff" for the Lear 25 wing that resulted in the 25G variant, with better performance at cruise and an increase in range of some 360 nautical miles.
There has been some talk that the problems of the Lear wings come from the FFA P16 design. Bill Lear's son has clarified that his father did not use the P16 as the basis of the Lear 23. The two aircraft use different, though similar airfoils.


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Why does a DME ground station wait 50 microseconds?
The FAA says that a DME ground station sends a response exactly 50 microseconds after having received an interrogation from an aircraft. For what reason is there a 50 microsecond delay?

An important question is: does it say (and mean) typically, or is that a specified delay?

if milliseconds is a typo, why doesn't someone edit it?

It would appear that the "typically" here is totally incorrect.  (See Henning below.)  Someone should edit it.

@JoeBlow: At Math.SE, where I spend most of my time, we're reluctant to edit questions to remove misconceptions by the OP. Clearing up the misconceptions should rather happen in answers, because getting that cleared up could be the entire point for the asker (here the OP might be asking because he thinks 50 ms is a strangely long delay to be using -- who knows?), and having it pointed out in the form of an edit is much less visible than an actual answer. I don't know if the standards are differently on aviation.SE, but this is why I didn't edit.

that's an interesting policy; understood. pedagogically, it makes sense.  I'm troubled by Wrong Text going out in to google though, you know  :O

@JoeBlow As Henning's answer mentions, the delay can be reduced when used with ILS systems in order to make the distance appear to be at the runway threshold rather than at the actual antenna of the DME ground station, so 'typically' seems accurate. For exact details of the DME system, see chapter 4 from FAA AC 00-31A.

I'm afraid I don't agree Reirab, you'd never use "typically" to describe an exactly specified part of a system (when it's deliberately varied shorter you're still exactly "simulating" that precision spec-specified time).

the purpose of the 50 microsecond delay is to eliminate the possibility of uncoordinated operation when the aircraft is very close to the ground station

From AVweb - DME Basics

Addendum
I have not found a clear explanation for what is meant by "uncoordinated operation" in the context of DME. What follows is speculation:
If you look at patents related to DME, it seems this predictable delay ensured that the initial transmission from the aircraft would have been completely received before the reply was transmitted by the base-station and received at the aircraft.
I believe a DME response has to match the pattern of the DME interrogation so that the receiver in the aircraft can identify which response to apply timing to if multiple DME-equipped aircraft are in range.
The pulse spacing is 12 µs1 and this is the same order of magnitude as the round-trip transmission time at 1 mile. A single DME interrogation contains several pulses. It may be difficult to design ground station equipment that can start transmitting a matching response if it has not yet received the pattern of data it has to reproduce. The alternative would be to start transmitting the response as soon as possible after detecting the end of receipt of a valid DME interrogation.
There are delays in processing within the ground station equipment which may vary (at least from station to station depending on equipment) and which therefore need to be made constant for accurate distances to be calculated by the airborne receiver. Any variations would be more significant at closer distances.
It might also be easier to design airborne equipment if it does not have to start receiving the response before it has finished transmitting the interrogation.

References

US Patent 3226714
PPRuNe discussion

I believe DME was invented around 1944-45 but cannot find any patents that early. There are quite a few patents on this subject.

Although it does not provide the detail, this should be the accepted answer.  Specifically, the "uncoordinated operation" would occur if the transmitter was still sending the second interrogation pulse when the first reply was received from an aircraft at close range.  50 µs corresponds to approx 4 NM within which, without the delay, aircraft would show unreliable readings.

@Simon: It is not the correct answer. The DME interrogator on board determines the slant range by measuring the time for the signal to reach the ground station (transponder), plus the time for the ground station to send the response (call this one T), plus the time for the response to reach the on board interrogator. Then the interrogator which is interested only in the time for the signal to travel back and forth, needs to remove T. If T is variable (dependent of the station design) then it is not possible. Hence T must be standardized.

Actually, I've just realised this "reference" is just from a pretty crappy, 20 year old general-interest magazine article! Sorry I at first commented that it seemed to be factual.

Hi mins. Do you have an actual reference that states the 50ms is standardised.  What's the allowed error for equipment makers?  (if it's an ISO standard or something, it's inconceivable these days it wouldn't be given with an error range.)

@JoeBlow: The 50 µs (not ms) is standardized in ICAO Annex 10, volume I, section 3.5.4.4. (This doesn't seem to be publicly available, but there are pirated copies floating around if you google hard enough). The specified tolerance is ±1 µs (section 3.5.4.5), which corresponds to an error of 500 feet in the result. Most installed DMEs do better than that.

"ICAO Annex 10, volume I, section 3.5.4.4" that is awesome

should surely be in your answer

I didn't accept this answer, because the phrase "uncoordinated operation" seems unclear to me.

@HenningMakholm According to FAA Advisory Circular 00-31A, the 50 +/- 1 us number is outdated. This AC states a specification of 56 +/- 0.25 us and changes the definition to be from rising edge of the first pulse of the interrogation to rising edge of the first pulse of the response, rather than the second pulse of each. See paragraph 131 on page 22.

@reirab: Yes, the ICAO specification just provides minimum recommendations that apply globally, but a country is allowed to aim for better precision with its own navaids. DME/P as specified for the Microwave Landing System has even smaller tolerances, down to a few tens of nanoseconds. (Since one cycle of the carrier frequency is about one nanosecond, this must call for some serious RF engineering -- in addition to a more precise redefinition of when exactly the "rising edge" is!)

Note that the 56 µs is between the first pulses in Y channels only. Since the second pulses still have a separation of 50 µs in all modes (even though it's not primarily specified separation), I kept my discussion in terms of that in an attempt to avoid confusion.

As for the edit: The ground station must not begin transmitting until it has received both pulses and verified that they have the right spacing. Otherwise it might mistake extraneous pulses for interrogations -- each interrogation frequency is shared between an X-channel and an Y-channel, which are distinguished by the separation between the interrogation pulses. Even worse, the same frequency carries replies for a different Y channel, so two stations might end up responding to each other's pulses indefinitely unless they wait until they have decoded the second pulse.

the sentence from "AVWeb" is utter crap and should be deleted.  (I don't know why you haven't deleted it, RGB!)  the rest of the answer is fantastic.

@Joe: That sentence appears in several sources. When I can find something better I'll edit that part. Still looking.

The delay is not 50 milliseconds, but 50 microseconds.
It takes some time for the ground equipment to decode the incoming signal, decide it needs to reply, and fire up its transmitter. In order for the distance measurement to be reliable, this inevitable delay needs to be precisely defined and the same for all DME stations. It also gives the airborne interrogation some minimal time to shut down its transmitter and start listening even when it's right on top of the ground station.
50 µs must have felt like a suitable interval to standardize on, long enough for it to be reasonably possible to engineer the ground stations to meet it (given the technology of the time the design was made), yet short enough that one DME station can service an acceptable number of aircraft without needing to handle overlapping delays between different interrogations.
The standard delay also means that a DME station used for ILS can be located midway between the runway's thresholds and adjusted such that aircraft see the distance to the threshold rather than the distance to the physical antenna, by reducing the delay appropriately. (Such a DME does not give useful readings for aircraft not aligned with the runway).
Note, by the way, that the 50 µs is the time between receiving the second pulse of an interrogation pulse-pair and transmitting the second pulse of the resply. So the ground station actually has to start transmitting the reply less than 50 µs after it knows it has received a complete interrogation. For Y channels where there are 30 µs between the two reply pulses, the ground station actually has only 20 µs to react.

A small point: the overlapping issue would be irrelevant, I think. If the delay was (let's say) 20 seconds, it would be utterly trivial to still send each one out exactly 20 seconds after the relevant arrival, even if as it happened many were arriving at the same time. In other words regarding this issue: "handle overlapping delays between different interrogations", it's quite true the soft3are engineers would have to "handle" that but (A) it makes utterly no difference what the specified delay is, and (B) as it happens, it is trivial anyway.

{Any obscure racetrack bugs which would appear - would appear anyway regardless of the set delay.}

@JoeBlow: It is indeed possible to handle overlapping delay period if you have a computer to keep track of them -- not utterly trivial, because the exact timings of replies have to be executed with something approaching nanosecond precision, but certainly doable. However, remember that the system dates back to the 1950s. At that time you wouldn't have a computer around to control the ground station; the delay would be implemented with analog circuits. In that setting, keeping several different delays active at the same time would be an entirely different problem.

@JoeBlow: As for delaying 20 seconds, quite independently of making the ground station do it, that would place ridiculous demands on the clock stability of the airborne equipment. Each part-per-million of clock drift would cause a measurement error of more than 3 NM.

Hi Henning - re 1950, fair enoughL I have no clue what they had then.  (It seems strange that we're still using concepts from 70 years ago? Wouldn't this have been updated somehow?)  Just regarding the 20 purely-an-example seconds.  It's still unclear whether the signal is meant o come out precisely 50 ms afterwards, or is the point of the guideline that it should be "around" 50ms .. or indeed does it mean "up to 50ms, the faster the better".  Nobody has been able to answer this.

@JoeBlow: It is precisely 50 µs (still not 50 ms, which is one thousand times longer than 50 µs). The principle of DME is that the aircraft measures how long it takes for the reply to arrive, and multiplies that with the speed of light. If the ground station replies with less than the specified 50 µs of delay, the aircraft will think it is closer to the DME that it actually is. For example, responding 12 µs too early will lead to a reading one nautical mile too short.

@JoeBlow: As for "70 years ago": The existing protocol works, and changing it would require all aircraft to carry receivers for two different systems in a transition period -- it could be years or decades until the ground stations that implement the old protocols had all been replaced. The cost of this would be phenomenal, so a change won't happen unless it provides concrete benefits. Not just because "we might have chosen a different number if we started over from scratch today".

I see, it's precisely 50 ms.  (I am using here ms for microseconds, as in lol or btw.  I'm guessing you are using the "official" SI unit, which is also nice.) Thanks for clearing that up.  You can see that the "typically" in the question was wildly confusing!

Just purely BTW.  "short enough that one DME station can service an acceptable number of aircraft without needing to handle overlapping delays between different interrogations"  Note that: if you make D shorter you are less likely to get racetracks (simple statistics), but anyone building such a system, would, absolutely have to be able to handle racetracks. It's possible in the analog era (or even today) the policy is that you throw away racetracks (items that arrive "during D") - and that's fine if it's the policy - but it seems dubious that would be the policy.

I feel that the first sentence of your answer should be... "To clarify, the delay is precisely, and must be precisely, 50 µs."

@JoeBlow I would assume that the primary reason that the DME system is not designed to attempt to keep multiple overlapping timers to reply to different aircraft is because there is only one single radio channel available to each DME ground station. If it tried to keep multiple timers and respond to different aircraft it would run into two problems: 1) The reply pulses would probably need to overlap, which isn't possible and 2) The aircraft would both accept the first response, resulting in the 2nd aircraft indicating closer than it actually is.

@reirab: The aircraft is supposed to send out a stream of interrogations at randomized times and only believe replies that consistently arrive the same amount of time after each interrogation. (The FAA document you found even specifies that a ground station must send out a steady stream of random "ghost" replies if it doesn't have enough interrogations to respond to, such that this feature in the airborne receivers will be exercised all the time).

Right, I was just about to explain this in terms of how "GT3" works to synchronise the players  :)  It's the same fundamental computer science concept used to (say) decide on "who should be the server" in something like Bonjour on your mac, or whatever.  Obviously today every aircraft device would just have digital IDs (hell, you'd just log in to a web server on the ground station :) ) .. engineers don't have to be clever like this any more.  It's a bit like, "traditional" automatic gearboxes are really clever; now they're just robotized, which is nothing.

@HenningMakholm Yes, that's true. That would solve the second problem of an airborne transponder not knowing whether a given reply was for it or not, but it still doesn't resolve the issue of there being only one channel on which a given DME ground station can transmit, which still generally limits it to only talking to one aircraft at a time. It can't send a response pulse to two aircraft at once (without doing some kind of multiplexing whose complexity would not have been worthwhile back then.) Even now, though, channel arbitration is still an issue all wireless protocols must solve.

@reirab: The channel arbitration protocol is that every aircraft sends interrogations at random intervals and is supposed to be resilient enough to keep functioning if some of them are lost due to collisions. It takes less than 0.1 ms for the ground station to respond to one interrogation and be ready for the next one, so as long as there are less than about 100 aircraft actively using a given DME (with at most 30 interrogations per second each, once they're in "tracking mode" and have some idea of which range to expect), collisions will be rare enough to cope with.

@HenningMakholm Right. That's why I said it wouldn't have been worthwhile to attempt a more complex arbitration scheme using some form of multiplexing. The current "don't respond to the second interrogation" scheme was good enough and much more simple to implement.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why do the in house Boeing 737 max tails have a drag chute?
In the recent videos of some in-house (tagged as Boeing instead of some airline), at the top of the aircraft's tail, there is a rope with a small drag chute attached to it. What is the purpose of this?
The drag chute setup can be seen at 0:15 and 0:30 into this video:

Which itself is a dupe of https://aviation.stackexchange.com/questions/14237/what-is-the-purpose-of-the-ball-and-string-attached-to-the-tip-of-the-vertical-f So indeed it appears to be an instrument.


text:
What is the FAA's definition of "for hire"?
(note: references to specific FAA documents or cases are always appreciated!)
This is a general question and might best be considered as open-ended.
It is, however, posed for clarification of a few specific regulations:
91.205(c)(4) "if the aircraft is operated for hire, one electric landing light..."
91.409(b) "...no person may operate an aircraft carrying any person for hire .... unless within the preceding 100 hours..."
91.205(b)(12) "if the aircraft is operated for hire over water and beyond vliding distance from shore, approved flotation devices..."
There may be others regulations that reference the term in Pt 91.
The point is: do these regulations imply that neither landing lights, life jackets, nor any current 100 hr inspection are required for e.g. (1) a solo rental, (2) a rental with $\leq$ pro-rata contributing friends, (3) a checkride ? It is my understanding that none of these are not "for hire" flights but perhaps there is clarification otherwise.
i.e.1) what flight scenarios/activities are conducted under for hire circumstances and therefore must comply with the above regs?
i.e. 2) Does the term for hire describe the airplane or the pilot?

See also: http://www.boldmethod.com/blog/2014/05/100-hour-inspections-explained/ @mongo

Without combing through the regulations, and the vast collection of LOI on the subject, I will give a general answer.
For hire for the purposes of 91.409 applies to the carriage of persons. So the carriage of a student pilot, with a flight instructor is "for hire."  The carriage of people receiving a scenic tour that they paid for, is "for hire."
If you belong to a flying club, essentially operated as a partnership, then the aircraft is not for hire, as the fixed and variable expenses are shared among the members of the club.  Essentially, you are paying your share for your aircraft, assuming the club or partnership owns the aircraft.  In that case the landing light and 100 hour examples do not apply.
In general, if operating part 91, and the pilot is paid in furtherance of the part 91 entity, then for hire does not apply. This would include for example a corporate pilot, paid by the owning entity of the plane.
In general, flight instruction is not considered a for hire operation, when the person receiving instruction is one of the "owners" of the plane, as in a partnership or flying club.
So, you have some examples for your second to last question.
For the last question, it covers the operation, and the purpose of the operation. So your paid pilot flying your TBM taking your family to your private island, is not for hire.
There are no less than 20 FAA Letters of Interpretation which have been written on this topic. The interpretations have even impacted states and how they charge sales and use tax.
Addendum #1: For the FAA, under 91.409, the term "for hire" applies to the carriage of the aircraft.  So if a flight school owns a plane, and a student gets instruction in the aircraft, it is subject to 100 hour inspections, because the student is being carried by the plane and CFI which the flight school is providing.

I don't think a rental plane is a "for hire" operation.  The business in that case is short term rental of equipment, not flying services.  I may be wrong but I'd like to see support for the idea that a rental to a non-owner pilot is a for hire operation.

@acpilot, normally "rentals" are available after a "checkout" normally provided by an instructor. The instructor charges for his time. That makes the flight a hire flight. If the instructor is hired by the aircraft owner, then the flight is not a "for hire" flight and there is no requirement for a 100 hour inspection.

To be clear:  If the aircraft is used ONLY for rental, and no flight instruction or carriage of anyone for hire is done, then there is no need for the 100 hour inspection, even though the aircraft is being rented.

Would the "for hire" definition be extendable to cargo-carrying aircraft where there weren't "people" passengers, but instead, material, packages, or other exclusively non-live cargo?

I have looked for that and do not have a definitive answer.  Any cargo operation I did has 100 hour or progressive inspections. All the part 91 flying I did, except flying clubs and private owners had 100 hours or progressive inspections.  All of the notes and case law I have reference "persons."  I have heard people talk about "for hire" for cargo, but have not seen any authority. Sorry.

If you are being paid to haul cargo then you are most definitely "for hire"...

Re-read 91.4099(b) "Except as provided in paragraph (c) of this section, no person may operate an aircraft carrying any person (other than a crewmember) for hire, and no person may give flight instruction for hire in an aircraft which that person provides" Which that person provides is the key detail. If the CFI owns the airplane then yes it is "for hire", if it's a rental from a flight club then 91.409(b) doesn't apply and 100 hour inspection is not required.

Not all flight clubs are shared ownership interest.  Some are commercial operations where membership permits discounted use or simply use.  Since there is not a equal shared ownership, it is my understanding that the FAA's position is that 100 hour inspections are required. Again, there are numerous LOIs on this issue.


text:
Are the vortex generators on the Cessna Caravan necessary and sufficient for safety of flight?
I've been doing some reading about the C208B (Cessna Caravan) in prep for my new job flying cargo; I came across this article which (as far as I can tell) is a transcription of litigation surrounding the caravan crash in Dillingham, AK 2001*.
In it there is an interesting discussion of the design integrity of the Caravan's horizontal tail: specifically, are the installed tail vortex generators necessary (or even sufficient) to maintain safe, level, and controlled flight within a reasonable envelope of flight conditions?

Cessna 208 Caravan tail with vortex generators right ahead of the hinge line (picture source)
I'm no aerodynamicist, so please correct any mistakes in my reasoning:
When center of pressure, $C_p$, is forward of the center of gravity, $C_g$, then there necessitates an up-force on the horizontal tail in order to maintain level flight.
From the POH (pdf pg. 29) the aft $C_g$ limit of a stock 208B is 40.33% MAC aft the leading edge; whereas $C_p$ is typically located about 25% MAC aft the leading edge. This would presumably generate a significant nose-up moment and demand proportional upforce on the horizontal tail surface. At higher AOAs the $C_p$ would be even farther forward of the $C_g$ and, as the above article discusses - in specific such flight configurations its possible that the tail is actually in the downwash of the wings, which could disturb airflow across the tail and therein fail to meet such demands.
So, the questions I have are:
(1) How typical is it that aft-$C_g$ limits are behind the $C_p$, such as in the 208? Wouldn't this imply longitudinal static instability?
(2) How do engineers typically design tails to maintain necessary up-forces in such cases? Are vortex generators viable solutions or are they, perhaps, after-thought band-aids to remedy an inadequately designed tail? Does anyone have more published references about the caravan's tail specfically?

You don't happen to have a picture or sketch of those vortex generators, so I can get a better idea what they are for?

https://drive.google.com/file/d/1T1ONL3PVD-vhM5wOhBIuKlE-IEE2sHiR/view?usp=drivesdk

@PeterKämpf it's interesting what you mention about weight & balance envelopes. Indeed the 208s typically seem to have an STC for increased MGW; I wonder if the vortex generators are req'd by such STC(s)

Thanks for the picture! If you don't mind I can insert it into your question. And yes, those help to avoid early separation at positive deflection angles of the elevator. Interesting.

Yes that's fine @PeterKämpf. Out of curiousity: what did you study in grad school and what do you do now?

I studied aerospace engineering and worked in that field for more than a decade, until 2000. I am now working on an IOT device in a small company and can tell you that this is much more satisfying than working in a large company. I am really poor at large-company politics, you know. If you wonder what I mean, read some Dilbert. This is like a documentary.

You need to keep several things apart:

The center of pressure is where all vertical aerodynamic forces can be summed up. It does not have a fixed location, but its location changes with angle of attack.
The aerodynamic center (or neutral point) is indeed at about 25% of the chord of a wing. Its location is fixed as long as the wing planform does not change.
The neutral point of the whole airplane is the weighted mean of the local neutral points of all horizontal surfaces. This is the most important neutral point for longitudinal stability.

In flight you will shift the center of pressure such that it is at the same longitudinal station as the center of gravity. You do this by regulating the tail lift or downforce with the help of the elevator.

When center of pressure, $\text{c}_p$, is forward of the center of gravity, $\text{c}_g$, then [this] necessitates an up-force on the horizontal tail in order to maintain level flight.

This is true if you speak of the wing's center of pressure. In a conventional configuration like the Cessna 208 the wing's center of pressure is ahead of the airplane's center of pressure.

How typical is it that aft-$\text{c}_g$ limits are behind the $\text{c}_p$, such as in the 208?

This is quite normal. No need for concern as long as the center of gravity is still ahead of the airplane's neutral point.

Wouldn't this imply longitudinal static instability?

No. Stability is reduced compared to a more forward center of gravity location, but still positive.

How do engineers typically design tails to maintain necessary up-forces in such cases?

The biggest concern of engineers in such a case are too light control forces. The force feedback loop is important for avoiding control deflections which could overstress the structure. Since the tail is in the downwash of the wing and carries less load per area than the wing, it is very, very unlikely that it stalls first. Without knowing the details, I am quite sure that the vortex generators were not added to cure a deficiency which occurs with a rear center of gravity location. Now I did some background reading and think that the vortex generators are needed to avoid early separation on the elevator in case of low-speed flight with flaps retracted and a rear center of gravity.

Are vortex generators […] after-thought band-aids to remedy an inadequately designed tail?

Yes, they are. They normally cure one corner of the envelope where something went out of whack during flight testing or after a modification (say, an increase in gross weight or in the permissible center of gravity range).

Does anyone have more published references about the Caravan's tail specifically?

Sorry, not me.

Now I have read the accident report and the linked transcript about the Caravan crash on October 10 in Dillingham, AK, in 2001. These are my remarks:

The witness in the transcript is very knowledgeable and I agree with all what he said.
His statement about the tail load change upon flap retraction seems spot on: Even though the accident report is inconclusive about the flap position, it seems likely that retracting the flaps coupled with insufficient lift on the tail made the aircraft pitch up and stall.
All the accident report has to say about de-icing focuses on the wing, but since lift-off speed seemed normal, I don't think ice on the wings was a problem. But what about ice on the tail? There is no mention of that.
Vortex generators on the upper side of the tail are unusual and a "red herring" about late fixes for insufficient pitch down control authority. Add some ice on the upper surface of the tail, and the fix will not be sufficient and cause an uncontrollable pitch-up with a rear center of gravity location.
That the pilot seemed to have pulled on the yoke on the way down did not help, either. Lift loss on the wing would cause a nose-down pitch change, but the aircraft remained stalled throughout its descent. Yes, it needs some effort to pitch down when the flight path is already 40° down, but this is the only way to avoid a crash. Remember AF447.

My conclusion from all this is that the Cessna 208 Caravan is stable and safe to fly as long as the tail is in pristine condition. Any contamination of ice and even de-icing fluid will compromise its ability to produce the required lift in low-speed flight with flaps retracted and a rear center of gravity.


text:
Have attitude indicators ever had triple failure in a commercial aircraft?
I'm wondering about couple of well known crashes (mainly AF447 and AirAsia 8501) and thinking: is there any case where triple redundant attitude sensor failure has ever occurred? In those accidents it seems that the pilots assumed that they cannot trust the indicators and forced the airplane to stall to ground.
I understand that air speed sensors can have triple failure (e.g. AF447 had such a situation for a minute) because those depend on measuring external environment. However, the attitude indicators are fully closed system which doesn't require any external inputs. As a result, having failure of three fully independent attitude sensors at the same time looks impossible to me.
Why do commercial pilots keep ignoring the attitude indicators in case other instruments are assumed to have faults?

Why do you say "pilots keep ignoring the attitude indicators"? The attitude does not help to prevent a stall because it does not show angle of attack.

Moreover, basic flying techniques would have saved both of those aircraft. Massively oversimplifying: the crews in those cases were staying inside the bubble of semi-automation / assistance rather than going back to basics of power + attitude to recover. This is understandable considering the mindset that is desired for that type of flying, but frustrating to those of us looking back in hindsight.

It also doesn't tell us anything about the relative effectiveness of those two approaches. If it is customary to rely on "semi-automation" on such flights, we have no real idea how many lives that might also have saved when "basics" would have failed. Hindsight rarely shines a light on things which did not happen.

I was using the expression "pilots keep ignoring the attitude indicators" because if the plane is at e.g. 30000 ft and you're not sure if AOA sensor or speed sensors have failed, the fact that ALL attitude sensors failing simultaneously is so low that you could just try to put the plane in nearly horizontal position and wait for a bit. Even if that doesn't result in fully correct AOA it would be much better than pushing or pulling the stick in panic. A plane at 30000 ft in horizontal orientation and engines running is not going to suddenly fall from the sky.

@DanSheppard maybe, but to echo what Mikko said: lowering the nose and applying power would likely have saved both of those aircraft irrelevant of what was being displayed on the instrumentation. The basics don’t generally fail, hence old adages like ‘speed is life’ that have been around since the dawn of aviation. I’m not anti-tech BTW. I’m anti-“not being able to cope appropriately when the tech fails”.

I'll also remind that this question is specifically about attitude indicators failing. In case a pilot has a situation where he or she can quickly figure out that at least some instruments have failed, it would be good to have some kind of idea how common is it for specific instrument type to fail to increase the speed of recovery. Also, learning to properly fly an Airbus plane in alternate law would be vital because it seems that pilots failed to do that in both above mentioned accidents.

Regarding the age old tradition of power+attitude: if pilots assumed attitude indicators had failed (only reasonable explanation for ignoring them), how do they reference their attitude during pitch black night in IMC? That's right, they don't. Fully agree on the importance of basic flying skills, but kinda hard to aviate with no airspeed, AoA data, and not trust in attitude indication.

Basic flying in a big jet consists of Bodyangle and Thrust, I don't see much other scope in IMC/dark night.

Yes! More or less, it depends on your definition of failure (see later).
As you said, the probability that three Inertial Reference Units (IRUs) fail is extremely rare. This is because IRUs are self-contained, internal to the aircraft, and independents. Instead, air data probes are subject to ice and external factors (for instance, bird strikes), thus more subject to faults. The IRS failure probability is so low, that in the non-neo A320/A330 the ECAM message "TRIPLE IRS FAILED" doesn't even exist (it shows all the three messages: IR1+IR2 FAULT, IR2+IR3 FAULT, IR1+IR3 FAULT).
However, yes, it happened: This is a real picture of an A330-220 (F-GRSQ) flying at FL240 with all IRUs failed in 2014:

According to the BEA report of the F-GRSQ incident (published in 2018), only 2 triple IRU failures incidents in Airbus aircraft happened since 1999: this one and another one in 2002.
In both cases the reasons of the triple failure are unclear. However, the BEA report suggests that the triple failure was triggered by erroneous pilots actions responding to a single IRU failure. In the F-GRSQ the possible explanation is that the pilots identified IR2 drifting too much and decided to switch it to ATT mode (degraded mode, providing only attitude), but they erroneously switched IR3 to ATT. Hours later, when IR2 drifted too much, Autopilot and FBW computer detected the data inconsistency and started to consider IR2 as failed, while IR3 was in ATT mode (and then not used by AP/FBW). Probably, at this point, by following a non-standard procedure, the pilots tried to restart all the IRs to regain the position on Navigation Display, including the  working IR1, which then lost alignment and caused the triple failure condition. Unfortunately, the lack of CVR recordings didn't allow the verification of this theory, but no malfunction was found in IR1 or IR3.

Why do commercial pilots keep ignoring the attitude indicators in case other instruments are assumed to have faults?

I don't think that pilots thought it was an IR problem in the sort of incidents you mentioned.

Great answer! I would call this pilot induced failure of triple redundant attitude indicator.

After occurrences such as AF447, industry study has concluded that

the "startle factor" comes into play during these 'Upset' scenarios.
Formal study of aerodynamic theory is, for most pilots, a thing from the distant past. There develops a reluctance to make simple aerodynamic interpretation.
Apart from this gawk factor there's a bit of "this can't be happening', because of the super safe technology we have become programmed to believe in. (and statistics mostly prove this is true.)
The fact that the airplane reached  the stage of approach to/stall indicates a severe loss of situational awareness, and a late recognition of such a fact can fluster the pilot, rendering her/him unable to make the required decisions nor take recovery action. In the AF 447 case, the 2 FO's were bouncing around in severe turbulence, and icing, with ECAM messages galore and without the PIC in the cockpit.

Typical Upset scenarios are not caused by failures of the IRS/attitude reference and modern technology and system architecture do not generally give unreliable info, they either work, or they don't, not anything in between.
Let's assume the case of triple IRS failure, it's surprising that we haven't referred to the 'standby' flight instruments. These include attitude and heading from independent sources and are DC driven (= will continue working on loss of all AC elec power, ie batteries only). The point being made is that it is very rare to have no attitude data. Procedures are based on comparing the LH and RH PFDs to the STBY, and taking the 2 that match to be correct and "switching out" the other.
The conclusion is that ICAO has mandated UPRT, Upset Prevention and Recovery Training, and this is probably the only way to escalate the problem sufficiently in the minds of pilots. Pilots have been lulled into a strange state of ennui, due to the genuine overall improvement in safety due advances in technology and procedures.

TL;DR: pilot training is not adequate and as a result, modern planes fly by magic according to an average pilot. When the magic fails, the pilot goes in panic mode.

Clarke's First Law: Any sufficiently advanced technology is indistinguishable from magic

Just wait until we have a large population of self-driving cars on the road, "piloted" by drivers who were never trained well in the first place (I'm looking at you, US driver training standards), and now haven't the slightest clue what to do when their autopilot fails.

@FreeMan I'm assuming that will be a different paradigm, one I haven't even thought of thinking about so far . . . . but come to think of it, that autopilot can fail too. They'll probably be able to achieve a lower mortality rate as compared to human driven cars just like the much quoted probability of death by aircrash is lower than that of a pedestrian crossing the road.

My point is that despite the incredible capabilities of autopilot, regulations require highly trained pilots and frequent retraining of pilots in the cockpit of every aircraft. Drivers (in the US, at least) are not highly trained in the first place (I know it's much better in other countries) and there are zero refresher courses required. Ever. Autopilot will fail, be it in an aircraft or road car. In the plane, there's someone who can take over. Average Joe in his car who hasn't actually driven in 4 years won't have a clue what to do...

Agree with you @FreeMan, here's some brain-food for all of us https://www.iata.org/contentassets/c0f61fc821dc4f62bb6441d7abedb076/ebt-implementation-guide.pdf

Granted, an aircraft can have upwards of 500 people in it, while a car crash might involve two fully loaded 15-passenger vans or a school bus with 40-60 people, but the recent videos of accidents in Texas during their once-per-decade (or so) ice storm shows that there can be significant pile ups involving potentially hundreds of people.


text:
Can I log multi-engine time abroad, when I'm not rated for multi in that country?
I hold a multi-engine FAA CPL and want to fly a multi-engine aircraft in another country. The aircraft is not N-registered and I don't have a multi-engine rating in that country. Can I log the time in my FAA logbook? I'd like to log the hours under my FAA license.
More Details:
Country: Colombia (ICAO)
Role: Second in Command
The plane is a multi, I don't have multi license from the registration's country but I have FAA Multi license, so the idea is to fly it and log the hours using my FAA license can I do that ? Even if I don't have the license to fly that plane from the country of the registration ?

I'm a little confused by the last part, how are you going to fly the plane in another country, but you can't legally fly it in that country? In what role are you flying? PIC? SIC?

It also matters what country you are flying in, you may only log hours in foreign registered aircraft if that aircraft is registered (and airworthy) in an ICAO member state. The short answer is that you would log the hours as if you were flying in the US, no different. The "wrench" is your last sentence and what it really means.

This seems like a similar case to this one.

A few more details might be useful: which other country is it; do you have a license there too; is your aim to log PIC time?

I voted to re-open: the question is specifically about logging time for which the pilot is rated in the US but not abroad. The other question is about how to physically record/manage information in both US and foreign logbooks. Those are completely different things.

If the a/c is not N-registered and you don't have the rating from that country that probably means you're acting as PIC or SIC illegally. If the country allows you to operate the aircraft then I see no reason why you can't log the time. There is a place in your logbook to put the a/c registration, US or otherwise.

Do you have a Colombian license of any kind, or not?

Yes I Have, I Hold Colombian Comercial License...But Single Engine

FAR 61.3 stipulates what is required to fly N-registered aircraft outside the united States.  There is nothing about flying non N-registered aircraft outside the U.S.  Rightfully so, the FAA does not have jurisdiction in that case.
To answer your question, you will need to reference that country's rule.  If they are an ICAO country, the rules are probably very similar.


text:
What does the letter D on the underside of the wing stand for?
Below is a picture of the Junkers F.13.
What does the letter D on the underside of the wing stand for?

(Image Source)


text:
Why do airplanes use MAYDAY when in danger but ships send SOS?
I thought SOS means "save our souls",  but apparently it doesn't.
But ships send SOS when in danger and they used Morse code.  Why do airplanes use MAYDAY?

SOS is not used since almost 20 years https://en.wikipedia.org/wiki/SOS : "SOS remained the maritime radio distress signal until 1999, when it was replaced"

No it doesn’t stand for save our souls.

Of interest from that wikipedia article, SOS is actually a mnemonic for a 9 element sequence.  It's not actually 3 letters, but rather "The SOS distress signal is a continuous spaceless sequence of three dots, three dashes, and three dots."  As such, other letters could be used as a mnemonic, such as VTB.  To distinguish between SOS and VTB, one would need to add spaces between the letters which are not there as part of the distress signal.

Back formation or not, SOS doesn't mean "Save Our Souls", if for no other reason than that (if you follow certain religions, anyway) sailors' souls would be in far more peril while ashore than at sea :-)  It means "Save Our SHIP".

This question is based on a false premise.  Ships also use "Mayday".  This reference is from the US Coast Guard, but the terminology is also standard in many other countries (Canada, for example).

Even ships use Mayday.

SOS sounds the same as spanish eso es, meaning that is or that’s true. Using it in voice transmissions would cause a lot of confusion in many parts of the world. Maritime VHF radio is still performed to a great extent in the languages of the reachable coastline, not at all only in English.

BIt doesn't mean "save our ship" either. As one of many articles that states this, see: http://www.boatsafe.com/nauticalknowhow/060199tip6.htm

@Penguin: That depends on what you mean by 'mean', doesn't it?  "Save Our Ship" is the meaning that people commonly assign to those letters.  As I said, it could be (and probably is, per your link) a back formation, but it has a perfectly sensible meaning.  "Save Our Souls" is meaningless, given any common definition of soul.  But further comment might be better suited to the English site.

I'm voting to close this question as off-topic because it's based on a false premise. Both aviation and marine use Mayday, and voice communication.

@jamesqf. I can't imagine any other interpretation of "mean"? A "soul" is defined as a person, or their spirit or essence. So to "save our souls" means "save us". Which is more meaningful than "save our ship" which implies saving the structure of the vessel, itself. But both seem to be "back formation".

@Penguin: Well, this is becoming a discussion more suited to a religion site, but I'll let Shakespeare speak for me,  "Why, what should be the fear?/I do not set my life at a pin's fee;/And for my soul, what can it do to that,/Being a thing immortal as itself?"  At least if one follows Christianity, all a shipwreck would do would be to kill the sailors' bodies, leaving their souls to go to Heaven or Hell.

The difference here isn't between ships and aircraft: it's between Morse code and voice.
The SOS signal is only for Morse code. It's short, easy to send, and easy to recognise. But it's not as convenient to say. It doesn't actually mean "save our souls". The letters were chosen just to form the simple Morse pattern, and "save our souls" is a backformation: it was made up by sailors later, partly as a joke, partly as a mnemonic.
"Mayday" is an English-looking spelling of French m'aidez, "help me". Spoken out loud, it's short, easy to send, and easy to recognise. It doesn't have any sounds that some nationalities can't say (such as r, th, or v). It's a good signal to use as voice, but would be much worse as Morse code, because it's too long.
Back in the days when aircraft were equipped with Morse code transmitters, they would have sent SOS as a distress call, just like a ship. And a ship with voice radio would send "Mayday" instead of SOS. Now that Morse code has fallen out of use, SOS is also disused. You only hear it in movie plots where the plucky hero doesn't have a working radio but can somehow improvise a way of signalling Morse code (usually by holding two wires together on a broken radio).

Comments are not for extended discussion; this conversation has been moved to chat.

many Chinese can't spell it because there's no /d/ sound in Mandarin, only /t/ and /tʰ/. Actually there are only voiceless consonants in Mandarin

@LưuVĩnhPhúc True (and I would in general take issue with “any sounds that some nationalities can’t say”), but that’s also true of German; and because in English voiced stops are generally unaspirated and aspiration is not distinctive, native English speakers will tend to interpret unaspirated voiceless stops as their voiced counterparts. Meanwhile, Spanish /d/ is realized as /ð/ in most dialects. In other words, the sound transcribed in Pinyin as “d” is no farther from the original French than many European pronunciations, and just as comprehensible, at least to Westerners.

'"Mayday" is an English-looking spelling of French m'aidez, "help me"'. I often read this, but as a French I'm very skeptical about it, it's grammatically incomplete and unnatural. "Help me!" translates to "aidez-moi !" imperative tense". If I shout "m'aidez", nobody will understand this is the verb aider (btw "help!" translates to "à l'aide !"). It could be an incomplete sentence for "I need your help!" ("il faut m'aider !" the infinitive, grammatically correct). -ez and -er endings have the same pronunciation.

@LưuVĩnhPhúc I speak Mandarin and, while this may be technically correct, in practice it doesn't matter. Mandarin speakers have common words that sound extremely similar to "may" (eg. 没 -- meɪ̯³⁵) and common words that sound extremely similar to "day" (eg. 得 -- teɪ̯²¹⁴ -- which despite what it looks like in IPA is very similar to "d" in practice). I doubt that anyone would have trouble understanding what a Chinese speaker vocalising "没得" meant in the context of aviation.

Data point:  My wife is a native speaker of Mandarin and learned English as an adult.  She had absolutely no problem saying "Mayday".

Ships use Mayday.
This is the transcript of M/S Estonia disaster from 1994, a major passenger ferry sunk.
01:23.11
Estonia>    Europa, Estonia.
01:23.15
Estonia>    Silja Europa, Estonia.
01:23.19
Europa> Estonia, this is Silja Europa replying on channel 16.
01:23.27
Estonia>    Silja Europa *
01:23.34
Europa> Estonia, this is Silja Europa on channel 16.
01:23.55
Estonia>    Silja Europa, Viking, Estonia.
01:23.59
Mariella>   Estonia Estonia.
01:24.02
Estonia>    MAYDAY MAYDAY.
01:24.07
Estonia>    Silja Europa, Estonia.
01:24.10
Europa> Estonia, Silja Europa. Are you replying— calling Mayday?

Source: http://oona.windytan.com/estonia/
You can also listen the same in Youtube:

SOS is for morse code, Mayday for voice.

This was painful to listen to and read about, but thanks for sharing.

It was a very painful event, national tragedy for Estonia, Sweden and Finland. I remember seeing the head of rescue, Raimo Tiilikainen, cry on television. You don't see major officials cry very often.

Merely being a ham operator for 58 years, this is my understanding. "SOS" means nothing. It's easily sent and easily copied (understood), and it follows "CQD" which was also used on the Titanic. CQD is most aptly "COME QUICKLY DISTRESS" (the D has various meanings / uses).
The senior radioman on the Titanic told the jr. to go ahead and try the new call. As an aside; a young Welsh lad, about 15 being a young ham, heard the Titanic's distress call, sic (ITS A CQD DE MGY CQD DE MGY SOS AM SINKING HAVE STRUCK A BERG). ("DE" means from,- MGY is the TITANIC'S Marconi owned wireless station call letters). He copied the text and went to the local police. When he told them, they laughed at him saying sic. ("the Titanic is unsinkable, go home").
They were surprised the next morning to hear / read the news. MAYDAY is from a French word "French "m'aidez",  "mayDAY," [one word], and means in actuality - "help me". It is a short form of venez m'aider - come and help me'.
This first URL is a simulated spark gap transmission.

The second is an exchange between the Titanic and the Carpathia.

I guess "come quickly" for CQ is another backformation. https://en.wikipedia.org/wiki/CQ_(call)

MAYDAY is from the French "m'aidez" meaning Aid Me, or HELP. Try it in a French to (your language) translator and it becomes clear. Hard to believe no one came up with the right response.


text:
What is this aircraft with a big gun on its back?
It looks like there is a really big gun on the back of this aircraft.

https://www.instagram.com/p/BQVqnYCAlZU/
I found it on Instagram. What is this?

The "big gun" is a rocket engine.

@Simon, it definitely is not a rocket engine; this engine is air-breathing (pulse jet).

Looking at the better quality picture in ymb1's answer, it looks more like a scope to me...

@JanHudec Cough, cough, er, that's what I meant. ;) Was thinking of V2, which of course looks nothing like this.  I'll get my coat.

The "rocket engine"  is a Pulse-Jet engine     @JanHudec You beat me to it, but I am leaving the comment anyway, as a same format counter to Simon's comment

Now, if you want an aircraft with a big gun, I suggest the Hs-129 B-3


text:
Where to get spatial data for ATC airspace?
I am looking for free or open spatial vector data (geoJson, .shp, .kml, …) which maps the responsibilities of air traffic control in South East Asia, worldwide would be very nice too. SkyVector.com seems to have an excellent data source but its not open. any ideas? Cheers!

"which maps the responsibilities of air traffic control": Can you be more explicit in what you're searching for? Down to which level? See also: ICAO maps and ICAO viewer.

I think the OP is looking for ATC sector data. In other words, what data delineates between FIRs, center/control, and airspace, and perhaps even different frequency sectors. Clarification on what the OP is looking for would be helpful.

sorry for the confusion, non aviator here :). I am looking for polygons which represent the areas of responsibility for air traffic control. I added an screenshot to my question to illustrate that (andaman sea of the coast of thailand with the highlighted areas of my interest). thanks for the links, looks promising but might take me a while to dig through

Data about ATC sectors in easy to use format, such as shapefiles, is generally not publically available. Take it from someone who has spent hundreds of hours manually importing such data from PDF text files to various GIS applications. You can find a few individual data sources here and there via Google, but they are limited geographically and more often than not, outdated.
What you need to do is locate the aeronautical information publication (AIP) of your country of interest, and navigate to the En-route section. Airspace data is usually contained in section ENR 2.1. What you will find is a more or less well formatted PDF file containing textual airspace definitions (WGS-84 based coordinates). You will need to manually transfer this text data into which ever format you need, using your preferred text editor or spreadsheet application.

Can I please ask how often are these definitions expected to be updated?

@A_A Depends on what you want to use the data for. Airspaces can be changed as often as every 28 days

Thank you. Personal curiosity. 28 days is an impressively short interval, for me anyway.

Thanks for your interesting answer, I see if this is suitable for my purpose, which is doing maps (non aviation maps)


text:
What is the correct name of this aircraft subsystem in English?
I would like to know what the correct name of this aircraft susbsystem is in English:
(Source)
name: пилотажно навигационный комплекс
+/- literally translated it could be something like: flight navigation system.
I am looking for some sort of blockdiagram of this system of either an F-35 (my favorite) or otherwise a boeing airplane. A blockdiagram which components are used and how they communicate with each other. E.g.:
(Source)
Could anybody suggest me some good places to look for?

Link showing the reverse of the same component: http://ntlab-sk.ru/sitefiles/1/10/30085/2.png

@JonathanWalters thx for your answer. Do you happen to have a picture from the bottomside of the component? Maybe that will answer one of my questions...

No, sorry, I do not. That is the only other picture I could find.

It looks to be some kind of AHRS or IMU, but it is impossible to tell from the images. There seems to be some kind of part number on top, but it isn't clear.

@RonBeyer I guessed, based on the google translate result, that this was a flight management system. But really not sure though...


text:
ATC calling out regional airlines in traffic advisories: phonetic or "aircraft paint" airline used?
When ATC points out traffic that is a regional airline (e.g. Compass, Envoy, SkyWest, etc.), do they use the phonetic callsign of the airline, or do they use the callsign of the airline that they are operating as (how the plane is painted)?
The plane that a regional airline flies might be painted differently, depending on which airline they are flying for.
My thinking:
1) Pros of using the "aircraft paint": aircraft who are flying in VMC can spot each other more easily.
2) Pros of using the phonetic callsign: aircraft are on the same frequency in the same phase of flight, so they are more situationally aware if you use their phonetic callsign because they have probably already heard it in another transmission.

In addition to the answers already here, aircraft "paint" scheme is often called between multiple jets in order to differentiate if there are a few of them in close proximity (on the ground). They don't have to be regional, either. "Delta 1921, follow the United-colored Boeing 757 ahead and to your right." Delta 1921 would then know they are not to follow the Fedex-colored 757 but the United colored one.
In the air, paint scheme is often used when pointing out traffic. "Traffic 9 o'clock and 5 miles, south bound a Delta-colored Boeing 717 descending out of 4 thousand, report that traffic in sight."
Both of these situations help differentiate between other traffic the pilots might see.

In the air, they don't bother; if you're close enough to read the paint, you're probably dead. They do give the type (if known) so you have an idea of what size plane you're looking for, though.
On the ground, they definitely call the paint--when they know it. This can cause confusion when a plane's paint doesn't match its callsign. For carriers known to use multiple paint schemes, they just call the type unless they can see the paint themselves from the tower.

Depends on what you mean by “point out.” A point-out, in an air traffic sense, is a technical term, meaning coordination between two controllers where one controllers traffic will entered another’s airspace but control will not be transferred. In that case, call signs, positions and altitudes are used.
Calling airborne traffic to other pilots: direction, distance, direction of flight altitude and type of aircraft
On ground: all the above. Paint job of the airline, type, location... anything goes. I’ve used call signs as well, as in “you’re number 20 for departure, following an opposite direction xxx, call sign is yyy.

They would use the aircraft's actual call sign, ie. Endeavor or Brickyard. More often, however, ATC will just call the type of aircraft (737, A330) and its bearing and distance rather than say the airline it flies for. This is because often they are looking for traffic several miles away, and unless it's an all-white British Airways A380, you probably won't be able to tell the color anyway.

I must say this is from personal experience...I haven't actually looked in the AIM.

They never use the call sign, just aircraft type or generic description e.g. regional jet. You often know the callsign since when they call out traffic to you they give a corresponding call to the other aircraft.